Actually, that is not really a true Den phal hybrid...
This is probably Dendrobium Burana Green Star or a NOID that was bred to be it or like it.
If it really is Dendrobium Burana Green Star, then...
It is a complex Dendrobium hybrid that mostly contains species in the section Spathulata in its heritage.
It is only a tiny percentage Dendrobium in the section Phalaenanthe.
BlueNanta
With a complex hybrid like this, I don't really know if it truly has a dormancy or not. Nor am I sure whether it has a tendency to behave like a Dendrobium in section Phalaenanthe and express some behavioral traits that are unlike those in section Spathulata or not.
I can tell you what I know of Dendrobiums in sections Spathulata and Phalaenanthe.
Normally, Dendrobiums in section Spathulata do
not have a dormancy.
Normally, Dendroniums in section Phalaenanthe
do have a dormancy. In cultivation, this dormancy is often completely ignored.
Dendrobiums in section Spathulata do
not go deciduous unless they were heavily stressed. In other words, losing a large amount of leaves on these plants is not a normal thing.
Dendrobiums in section Phalaenanthe
can go deciduous during the winter period and this is normal.
This is a Dendrobium hybrid that is likely to handle intermediate to warm temperatures.
It likes indirect bright light.
They are able to handle a humidity range of 60% - 90%.
Hopefully this gives you a sense of where to go with your plant.
I would not be terribly concerned about losing those leaves. Being that a Dendrobium from section Phalaenanthe is in its heritage, the loss of that many leaves is not a terribly devastating thing.
Keeping it humid is a good way to make sure it will be ok.